The present invention relates to a thermal post-combustion device for the purification of exhaust air, comprising
a) an outer housing surrounded by an insulating jacket;
b) a combustion chamber bounded by a combustion chamber housing and arranged inside the outer housing;
c) a burner chargeable with a fuel and that comprises a burner nozzle and a first flame tube that surrounds the burner nozzle and connects the space between the outer housing and the combustion chamber housing to the combustion chamber.
Thermal post-combustion devices likewise serve in the same way as regenerative post-combustion devices for the purification of industrial waste gases that contain combustible substances. Regenerative post-combustion devices are employed in particular in cases where the purified gases are to be passed at as low a temperature as possible directly to a flue and the energy efficiency should be as high as possible so that the combustion process proceeds without the addition of external energy. This takes place through a relatively complicated heat exchange between the fed exhaust air and discharged purified combustion air.
Thermal post-combustion devices on the other hand employ a so-called xe2x80x9csurface burnerxe2x80x9d for the combustion of the impurities entrained in the exhaust air, to which burner external energy is fed in the form of fuel. These surface burners operate without fans and extract the oxygen required for the combustion from the exhaust air to be purified, which is supplied under pressure. Also, thermal post-combustion devices generally comprise a heat exchanger in which heat is extracted from the combustion gases so that the latter flow out at a lower temperature; some of the heat is fed to the exhaust air to be purified, with the result that this is introduced already preheated into the actual combustion process. In general process heat is extracted from a thermal post-combustion device for use in another heat-consuming procedure taking place adjacent thereto, e.g. for heating purposes.
With the known thermal post-combustion devices available on the market of the type mentioned in the introduction, the burner has only a single flame tube, through which the exhaust air to be treated is introduced into the combustion chamber and fed to the flame generated by the burner nozzle. Since hot and cold air do not readily mix, with these known thermal post-combustion devices the complete combustion of all impurities is complicated despite the use of air vortexing means, with the result that higher flame temperatures have to be used for the combustion. This is associated with a threefold disadvantage: on the one hand the energy consumption is high. Secondly, materials that can withstand relatively high temperatures have to be used in the device and, finally, more undesirable nitrogen oxides are formed due to the high flame temperature.
The object of the present invention is to modify a thermal post-combustion device so that a complete combustion of the impurities in the exhaust air takes place already at relatively low flame temperatures.
This object is achieved according to the invention if
d) the burner has at least one further (xe2x80x9csecondxe2x80x9d) flame tube that is arranged completely within the combustion chamber and the end of the first flame tube lying within the combustion chamber is surrounded by the further flame tube of larger radius so that an annular gap is formed between the first flame tube and the further flame tube.
The design of the burner according to the invention permits a circulating flow within the combustion chamber itself, the circulating flow passing through the gap between the first flame tube and the second/further flame tube and being assisted by the suction effect generated by the gas flow streaming through the inner flame tube. The exhaust air to be treated accordingly does not pass through the combustion chamber in a single passage, but is guided, possibly several times, through the flame of the burner nozzle before it finally leaves the combustion chamber in the direction of the heat exchanger. The circulating flow confers several benefits: there is a better air vortexing and thus mixing of cold and hot air streams, which improves the combustion. All the regions of the whole combustion chamber are heated uniformly. A complete combustion is ensured due to the multiple passage of the combustion gases through the flame. Overall it is thereby possible to reduce the flame temperature without impairing the complete combustion. Tests have shown that considerable energy savings of up to 10% may thereby be obtained. Also, cheaper materials may be employed for the various structural elements of the thermal post-combustion devices since they are not exposed to such high temperatures.
Particularly preferred is that modification of the invention in which a deflection means is provided spaced from the outlet opening of the further flame tube in the radially outer region of the combustion chamber, which device redeflects combustion air incident on the latter along the wall of the combustion chamber housing in the direction of the annular gap between the first flame tube and further flame tube. The deflection means thus assists the circulating flow mentioned above since it prevents the greater part of the air from already leaving the combustion chamber during its first passage through the flame.
It is furthermore convenient if the burner nozzle comprises a nozzle housing provided with passage openings and a fuel channel that has, in the region adjacent to the outlet opening, a venturi-like cross-sectional profile. The flow velocity of the fuel can be increased by this venturi-like cross-sectional profile and foreign gases can be aspirated through the passage openings of the nozzle housing so that the energy content of the fuel is reduced by xe2x80x9cdilutionxe2x80x9d. The result is a flame of lower temperature that produces fewer nitrogen oxides. Also, the generated flame is broadened in the radial direction due to the acceleration of the fuel. This facilitates the introduction into the flame of the air flowing through the first flame tube and possibly through the air vortexing means located in the latter.